Re: Solving the data inheritance problem

"James Kanze" <>
6 Dec 2006 12:15:00 -0500
Kirit Sflensminde wrote:


For a class like std::map< key, value > we know that there are
constraints that the map implementation (and definition in the
standard) enforces on the abstract type 'key' and that we must conform
to in order to substitute any other type in its place. This has nothing
to do with sub-classing from some common ancestor.

Languages like Java which lack generic programming enforce the use of
hierarchies that switch an operational polymorphism design into
inclusional polymorphism implementation because the langauge lacks the
tools to express the design clearly (and Java interfaces are just a
(nasty) way of doing this). In C++ we don't have this constraint
because our language enables us to express the design clearly and
exactly to start with.

Attention about trying to apply a set of definitions in the
wrong place. Neither C++ (at present) nor Java have any real
means of expressing the constraints of the instantiation types
of a map. (There is a proposal to add constraints to C++,
precisely because the need for this was felt.)

The templates, in this case, enforce a different set of
constraints---they statically enforce predicates over the type
system. Something that was lacking in Java until recently. But
this has nothing to do with polymorphism.


The final point then becomes whether we choose to implement the
operational polymorphism through templates (as the STL does) or through
virtual functions. In C++ this becomes an implementation detail with
one input becoming how and when the client code that uses our classes
gets used and another being which forms of behavioural extension we
wish to support.

Since when is something which depends on "how and when the
client code uses our classes" an implementation detail. Using
templates for polymorphism means static resolution---a serious
restriction for the client, which should normally be avoided.
The main "simple" use of templates is to statically enforce
predicates over the type system: if I want a vector which will
only hold Foo's, and nothing but Foo's, I can use
std::vector<Foo>, and any attempt to insert or extract anything
other than a Foo will fail at compile time.

There are very few cases where you can reasonably choose between
templates and inheritance. They do different things, and play
different roles in programming. Any good modern language will
provide both (in some form or another).

James Kanze (GABI Software)
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